In food shops in convenience stores, department stores, super markets, etc., foods, such as daily dishes, noodles and salads, are placed in food containers, e.g., tray, cup or bowl, and sold. Such a food container is composed of a container body and a cover. The container body is, in general, manufactured by thermoforming a sheet of polypropylene, formed polypropylene, filler-containing polypropylene, polyethylene, foamed polyethylene, formed polystyrene, formed heat-resistant polystyrene, amorphous polyethylene terephthalate (A-PET), etc, by a vacuum forming machine, a pressure-forming machine or a vacuum-pressure-forming machine. The cover is formed from a sheet, such as A-PET, biaxially oriented polystyrene (OPS) or polypropylene (PP) (JP2005-329972A).
Recently, it is frequently conducted that foods packaged in a food container are bought and heated as it is by a microwave oven. When foods containing oils are heated together with the food container by a microwave oven, the temperature of the foods is raised to around 150° C. Accordingly, food containers are required to have a high heat resistance up to 150° C. Even in the case of food not containing oils, food containers for retort foods are required to resist a retort sterilization temperature of 125° C. Furthermore, food containers are desired to have a high transparency so that the foods packaged therein can be appreciated clearly at a look and can improve the commercial value thereof.
However, none of the above-mentioned conventional sheets have both a high heat-resistance and a high transparency. That is, A-PET and OPS have a high transparency, but do not have a high heat-resistance and soften at around 80° C. A PP sheet has a high heat-resistance but is inferior in transparency.